The invention relates to lighting systems which enable control of the color temperature of the light output, and which also takes account of the daylight level.
There is a general desire to use more daylight lighting in indoor spaces this to save energy on artificial lighting and to provide more natural lighting scenes. This development is called day light harvesting. Shop owners for example are interested to allow more daylight into their sales spaces, for example to save energy and to establish a green image for their brand. The problem however is that day light is dynamic and changes. The color temperature changes over day due to the position of the sun (more blue in morning and more warm in the evening time) and also the light level will vary over the course of a day in relation to the time of the day, the season and the weather conditions outside. Shop owners want to control their lighting conditions.
As the light intensity and color characteristic of the sun change throughout the course of the day, it would be advantageous to keep a given relationship between the general lighting within the shop and the directed lighting to maintain, for example, a given CRI, or other desired lighting characteristic over a display area within the shop.
In many environments, such as offices, factories but also living rooms, light is formed by a combination of incident daylight and added artificial light. In many cases the daylight cannot be influenced, or only to a limited extent, by the user, for example by opening or closing a blind. This makes control of the artificial light all the more important.
It is therefore known to provide a lighting system which takes account of the daylight level, notably for the artificial lighting of office buildings. In known systems, a light sensor is provided for measuring the daylight level, and a control signal is derived from the daylight level. A control unit is then arranged to switch on the artificial light when the measured daylight level drops below a predetermined minimum or, conversely, to switch off the artificial light when the measured daylight level exceeds a predetermined maximum. It is known in particular that in office lighting systems the control unit adjusts the intensity of the artificial light mainly inversely proportionally to the level of the daylight.
The sensors can be installed on the most important planes in the space to be illuminated. In a supermarket for example they can be installed on racks in the corridors. In a hospital, the sensing can be for the horizontal level of the patient's bed. By measuring the lighting level it is possible to control the general lighting level by dimming or increasing the artificial general lighting, to maintain an overall minimal lighting level.
It has also been proposed to provide dimmable and at the same time tunable white lighting. For example WO 96/28956 discloses a system which enables the color point of (fluorescent) lighting to be controlled in dependence on the ambient light level.
This invention relates in particular to lighting systems which includes general lighting units and accent lighting (by which is meant a more directed and generally higher intensity lighting). A problem arises that as the general lighting is changed, for example in response to the ambient light conditions, the effectiveness of the accent lighting alters. This can reduce the benefit of the accented lighting, either to provide a brighter workspace or to illuminate objects on show.